A rotary machine burns fuel in combustion chambers to provide energy to the machine in the form of hot working medium gases. The hot working medium gases are flowed to the turbine section of the machine. In the turbine section, airfoils form stationary arrays of stator vanes and rotating arrays of rotor blades. These airfoils are employed to direct the flowing gases and to extract energy from the gases. As a result, the airfoils are bathed in a hot working medium gases during operation of the engine causing thermal stresses in the airfoils which affect the structural integrity and fatigue life of the airfoil. These thermal stresses have been a constant source of concern since the advent of high temperature rotary machines, such as gas turbine engines, because of the need to operate the engine at high temperatures to maximize engine efficiency. For example, the airfoils in the turbines of such engines may see temperatures in the working gases as high as 2,500.degree. F. life of the airfoil by reducing the level of thermal stresses in the airfoil
One early approach to airfoil cooling is shown in U.S. Pat. No. 3,171,631 issued to Aspinwall entitled "Turbine Blade" . In Aspinwall, cooling air is flowed to the cavity between the suction sidewall and the pressure sidewall of the airfoil and diverted to various locations in the cavity by the use of turning pedestals or vanes. The pedestals also serve as support members for strengthening the blade structure.
As time passed, more sophisticated approaches employing tarturous passages were developed as exemplified in the structure shown in U.S. Pat. No. 3,533,712 issued to Kercher entitled "Cooled Vane Structure for High Temperature Turbines". Kercher discloses the use of serpentine passages extending throughout the cavity in the blade to provide tailored cooling to different portions of the airfoil. The airfoil material defining the passages provides the necessary structural support to the airfoil.
Later patents, such as U.S. Pat. No. 4,073,599 issued to Allen et al entitled "Hollow Turbine Blade Tip Closure" disclose the use of intricate cooling passages coupled with other techniques to cool the
extending passage in the leading edge region of the blade. The flowing air in the passage also convectively cools the leading edge region as did the passage in Aspinwall.
The cooling of turbine airfoils using intricate cooling passages and film cooling holes alone or in conjunction with trip strips to promote cooling of the leading edge region are the subject of many of the latest patents such as: U.S. Pat. No. 4,177,010 issued to Greaves et al entitled "Cooled Rotor Blade for A Gas Turbine Engine" (film cooling holes); U.S. Pat. No. 4,180,373 issued to Moore et al entitled "Turbine Blade" (film cooling holes and trip strips); U.S. Pat. No. 4,224,011 issued to Dodd et al entitled "Cooled Rotor Blade for A Gas Turbine Engine" (film cooling holes); and U.S. Pat. No. 4,278,400 issued to Yamarik et al entitled "Coolable Rotor Blade" (film cooling holes and trip strips). These blades are typified by large cooling air passages in relation to the thickness of the walls in the leading edge region of the blade.
Recent aerodynamic studies suggest that an elliptical leading edge has advantages in performance during operation of the gas turbine engine. The elliptical leading edge is used in conjunction with an airfoil that has a thinner cross-sectional shape (thickness to chord length) as compared with prior airfoils. Despite the thinness of the profile, a minimum thickness of the walls is required to provide structural support to the airfoil and to enable the airfoil to sustain a certain amount of statistically expected foreign object damage. The result has been the advent of a new airfoil having an elliptical leading edge for aerodynamic purposes and having thicker walls relative to the size of the cooling air passages in comparison to the relationship between the walls and the size of the passages in prior airfoils. In addition, in the interest of fuel efficiency, it is not desirable in certain stages of the turbine to use film cooling for the leading edge region of the airfoil.
Accordingly, scientists and engineers are seeking to develop coolable airfoils for use in high temperature turbines which efficiently use cooling air, which cool adequately the leading edge region of airfoils with narrow passages in comparison to the thickness of the airfoil walls and yet which avoid the discharge of cooling air through film cooling from the leading edge region of the airfoil.